feat: Linear OAuth 2.0 (#10851)

Fixes https://linear.app/chatwoot/issue/CW-3417/oauth-20-authentication
We are planning to publish the Chatwoot app in the Linear [integration
list](https://linear.app/docs/integration-directory). While we currently
use token-based authentication, Linear recommends OAuth2 authentication.
This PR implements OAuth2 support.

---------

Co-authored-by: Sivin Varghese <64252451+iamsivin@users.noreply.github.com>
Co-authored-by: Shivam Mishra <scm.mymail@gmail.com>

lib/integrations/linear/processor_service.rb

@@ -76,7 +76,6 @@ class Integrations::Linear::ProcessorService
   end
 
   def linear_client
-    credentials = linear_hook.settings
-    @linear_client ||= Linear.new(credentials['api_key'])
+    @linear_client ||= Linear.new(linear_hook.access_token)
   end
 end

lib/linear.rb

@@ -2,9 +2,9 @@ class Linear
   BASE_URL = 'https://api.linear.app/graphql'.freeze
   PRIORITY_LEVELS = (0..4).to_a
 
-  def initialize(api_key)
-    @api_key = api_key
-    raise ArgumentError, 'Missing Credentials' if api_key.blank?
+  def initialize(access_token)
+    @access_token = access_token
+    raise ArgumentError, 'Missing Credentials' if access_token.blank?
   end
 
   def teams
@@ -108,7 +108,7 @@ class Linear
   def post(payload)
     HTTParty.post(
       BASE_URL,
-      headers: { 'Authorization' => @api_key, 'Content-Type' => 'application/json' },
+      headers: { 'Authorization' => "Bearer #{@access_token}", 'Content-Type' => 'application/json' },
       body: payload.to_json
     )
   end